Self-cleaning inlet screen to an ocean riser pipe

ABSTRACT

A long, vertically disposed ocean water upwelling pipe, such as a cold water riser in an ocean thermal energy conversion facility, is fitted at its lower inlet end with a self-cleaning inlet screen. The screen includes a right conical frustum of loose metal netting connected at its larger upper end to the lower end of the pipe. A heavy, negatively buoyant closure is connected across the lower end of the frustum. A weight is suspended below the closure on a line which passes loosely through the closure into the interior of the screen. The line tends to stay stationary as the lower end of the pipe moves, as in response to ocean current vortex shedding and other causes, thus causing the closure to rattle on the line and to shake the netting. The included half-angle of the frustum is about 20° so that, on shaking of the netting, marine life accumulated on the netting becomes loose and falls free of the netting.

REFERENCE TO RELATED APPLICATIONS

The present application contains drawings and detailed descriptionswhich are very similar to those found in four related applications eachof which claims different aspects of the structure and proceduredescribed in the present document. These other applications are asfollows:

(1) Ser. No. 935,591 filed Aug. 21, 1978;

(2) Ser. No. 935,641 filed Aug. 21, 1978;

(3) Ser. No. 935,672 filed Aug. 21, 1978; and

(4) Ser. No. 935,673 filed Aug. 21, 1978.

Persons interested in the structures and procedures described in thepresent document, but not claimed herein, may find it useful to considerpatents issued on these other applications or on any divisional or otherapplications based thereon.

Also, such persons may find it useful to consider the descriptions andclaims of commonly-owned copending applications Ser. Nos. 886,904 and886,907, both of which were filed Mar. 15, 1978. Application Ser. No.886,904 pertains to a tensile core arrangement for a flexible, largediameter duct suspended in an ocean. Application Ser. No. 886,907pertains to a stabilizing mass connected to the lower end of an elongateduct disposed pendulously in an ocean.

BACKGROUND OF THE INVENTION Field of the Invention

This invention pertains to large diameter fluid flow ducts for use inocean engineering and the like. More particularly, it pertains to ascreen for the inlet end of the duct which is arranged to clean itselfin response to motions of the duct caused by ocean currents and othereffects.

Review of the Prior Art and Its Problems

Many sophisticated proposals have been made in the field of oceanengineering which call for the use of large diameter vertical ducts ofgreat length extending from at or near the ocean surface to lower endsunconnected to the ocean floor. These proposals include concepts forocean thermal energy conversion and for mariculture.

The ocean thermal energy conversion (OTEC) concepts propose to use thedifference in thermal energy levels between warm surface water andcolder deep water to generate electricity, for example. The availableenergy level diffenence is low and so these proposals rely on the use ofvery large quantities of warm and cold water, and call for the necessarylarge volumes of deep ocean cold water to be brought to the watersurface through very large vertical ducts of great length. Such ductsare sometimes called "upwelling ducts", "upwelling pipes", or "riserpipes".

Any structure which extends vertically for any significant distance inthe ocean will encounter at least one ocean current. As the currentsmove past the structure, such as a cold water riser pipe, a vortex iscreated on the downstream side of the pipe. The vortex will periodicallyseparate from the pipe, thus causing a lateral force to be applied tothe pipe. Regardless of whether the pipe is flexible or rigid in nature,such forces will cause the pipe to oscillate, and the lower end of thepipe will move to and fro.

Even at the great depths proposed as the location of the inlet ends ofOTEC cold water riser pipes, some marine life, principally animal life,will be present. Such life can enter into the riser pipe and move withthe upward flow of cold water into the power generating equipmentcarried by the surface floating structure. The presence of such lifeforms in the generating equipment would lead to a reduction in theefficiency of the equipment and possibly even damage thereto.

It is thus seen that a need exists, in an OTEC installation, for a wayto protect the inlet end of the riser pipe from the entry thereinto ofmarine life forms. The protection mechanism should not provide anysignificant impediment to the flow of water into the pipe throughout theentire long-term life of the installation. Because of the water depthsinvolved, the protection mechanism must not require service throughoutits life.

SUMMARY OF THE INVENTION

This invention addresses the need identified above by providing astructurally simple self-cleaning screen assembly connectible to theinlet end of an OTEC cold water riser pipe, for example, the protectthereby to pipe from the entry thereinto of marine life formssufficiently large to be troublesome. Because of its structuralsimplicity, the screen is economical. It relies upon the natural motionsof the lower end of the pipe to produce selfcleaning, of the screen.Because the screen is selfcleaning, it does not require service.

Generally speaking, this invention provides an inlet screen useful witha downwardly facing inlet opening to a pipe through which sea water andthe like is to flow. The screen comprises a conical frustum of loosenetting having a larger upper end and a smaller lower end. Means areprovided for connecting the frustum upper end to the pipe inlet opening.A heavy, negatively buoyant closure is connected across the lower end ofthe frustum.

Such a screen preferably is used with a weight suspended below theclosure on a line which passes substantially centrally through theclosure to the interior of the screen. The closure can rattle on theline and shake the netting and to fall away from the screen.

A screen according to this invention has utility in ocean engineeringapplications separate from ocean thermal energy conversion. For example,it can be used on a cold water riser pipe in mariculture in which coldnutrient-rich water is brought to near the ocean surface, as in thegrowing of kelp, for example.

DESCRIPTION OF THE DRAWINGS

The above-mentioned and other features of this invention are more fullyset forth in the following detailed description of the presentlypreferred embodiment of this invention, which description is presentedwith reference to the accompanying drawings, wherein:

FIG. 1 is a cross-sectional elevation view of an OTEC upwelling pipeassembly and the mechanism by which it is connected to a floatingstructure;

FIG. 2 is a fragmentary cross-sectional view taken along line 2--2 inFIG. 1;

FIG. 3 is a cross-section view taken along line 3--3 in FIG. 1;

FIG. 4 is an enlarged cross-sectional elevation view of the inlet screenat the lower end of the pipe assembly;

FIGS. 5 through 12, inclusive, are simplified elevation and perspectiveviews which illustrate various stages of a procedure for deploying thepipe assembly and connecting it to the floating structure; and

FIGS. 13 through 21, inclusive, are simplified elevation views whichillustrate various stages of a procedure for releasing the pipe assemblyfrom the floating structure and recovering and reconnecting a releasedpipe assembly to the floating structure.

DESCRIPTION OF THE ILLUSTRATED EMBODIMENT

An ocean thermal energy conversion (OTEC) facility 10 is shown inFIG. 1. FIG. 1 is in two parts, and the broken line which interconnectsthese two parts shows how they are related to each other.

OTEC facility 10 includes a buoyant hull structure 11 which floats onthe surface of an ocean at the desired location of the facility. Hullstructure 11 may be the hull of a ship, such as a T-2 tanker, modifiedto serve as a major component of the OTEC facility, or it can be astructure constructed specially for this purpose. The desired locationis at a place where the water depth is sufficiently great that, wellbelow the surface, the temperature of the ocean water is sufficientlylower than the surface water temperature to provide the thermodynamicenergy differential necessary to the operation of an OTEC facility.Typically, the water depth at the location where hull 11 is floated maybe on the order of 4000 feet or more. Facility 10 also includes anupwelling pipe assembly 12 which is connected at its upper end 13, via agimbal and ball joint suspension and coupling 14 and a quick-releaseconnector assembly 15, to hull 11. A retrieval assembly 16 is associatedwith the upper end of the pipe assembly. A stabilizing bottom weight 17is suspended below the lower end of the pipe assembly.

A well 18 opens through the bottom of the facility hull. The gimbal andball joint suspension and coupling assembly is located, in use, at thelower end of well 18, and pipe assembly 12 is disposed below the hullsubstantially coaxially with the vertical centerline of the well.

As shown in FIGS. 1 and 2, gimbal and ball joint suspension and coupling14 includes inner and outer gimbal rings 20 and 21. The outer gimbalring is pivotally supported relative to hull 11 by a pair of trunions 22which are coaxially aligned along a common axis 23 on opposite sides ofthe outer gimbal ring. Trunions 22 cooperate in suitable bearingslocated in the outer gimbal ring and in bearing blocks 24. Each of thebearing blocks cooperates between a pair of vertically extending guides25 secured to the adjacent surfaces of well 18; the bearing blocks arevertically movable in the hull along the guides. The "in use" positionof suspension and coupling 14 is shown in FIG. 1 wherein the bearingblocks are supported on bottom stops 26 secured to the hull to span thespace between adjacent pairs of guides 25 closely above the lower end ofwell 18. Suitable upper stops 27 are provided at the upper ends of theguides within the well to limit the upward extent of movement of thebearing blocks in the guides.

A pair of trunions 28 interconnect the inner and outer gimbal rings atdiametrically opposed locations on both of the rings. Trunions 28 arealigned along a second gimbal axis 29 which is perpendicular to andintersects gimbal axis 23. One of the gimbal axes lies in thelongitudinal vertical center plane of hull 11, and the other extendstransversely of the hull center plane. It is apparent that any structureconnected to the inner gimbal ring will tend to remain in a stableposition despite angular motions of the hull about either or both of thegimbal axes.

As shown in FIG. 1, the inner gimbal ring 20 includes a frame 30 whichdepends below the inner gimbal axis. An annular, downwardly open femalesocket member 32, an element of quick-release connector assembly 15, isconnected to frame 30 in such manner that the axis of the socket memberpasses through the point at which gimbal axes 23 and 29 intersect. Thesocket member is flared circumferentially of its lower end. Adjacent itsupper end, the tubular configuration of the socket member is taperedinwardly for a short distance along the length of the member intoconnection with the lower end of a hollow ball element 33 which has anouter surface defined as a portion of a sphere. Ball element 33 has itslower end disposed below the point of intersection of the gimbal axes,and has its upper end located above such point. The center of curvatureof the ball element is centered upon the point of intersection of thegimbal axes. The ball element is hollow and is open at its upper end.The inner surfaces of the ball element define a portion of theboundaries of a fluid flow path, other portions of which are defined bythe interior of pipe assembly 12 and by a cold water sump 34 defined inthe lower portions of hull well 18.

The ball joint aspect of suspension and coupling 14 also includes anouter sleeve element 35 which cooperates with the spherical outersurface of the ball element. Sleeve element 35 is carried by the hull offacility 10. Element 35 has a tubular portion 36 which extends along theaxis of the well from above and to below the point of intersection ofthe gimbal axes. At its lower end, tubular portion 36 carries a sealassembly 37 which cooperates with the spherical outer surface of theball element to provide a substantially liquid-tight seal. This seal,however, is such that the ball element can move relative to the sealassembly. The motions of the ball element relative to the stationarysleeve element 35 are angular motions about the point of intersection ofthe gimbal axes.

Element 35 preferably defines the floor of cold water sump 34. To thisend element 35, except for the presence of tubular portion 36 thereinwhich cooperates with ball element 33, is generally flat and has aperipheral configuration which conforms to the contour of well 18,including appropriate cutouts in its perimeter to enable it to cooperatewith bearing block guides 25. Element 35 is supported in its operativeposition on suitable support flanges 61 which extend inwardly from thewalls of the well around its circumference except between adjacentbearing block guides 25.

Pipe assembly 12 is of great length. In a presently preferred embodimentof this invention, pipe assembly 12 has an overall length, from itsupper end to the lower portion of a self-cleaning screen assembly 38located at the lower end of the pipe assembly, of 2185 feet. The pipeassembly is flexible along its length in order that the generation ofbending stresses in the pipe assembly due to the effects of oceancurrents can be minimized. Preferably the pipe assembly is defined bythree parallel lengths 40 of polyethylene pipe having a nominal diameterof 4 feet and a wall thickness on the order of 2 inches. A steel cable41 extends within each individual pipe length over the entire elongateextent thereof, as shown in FIG. 1. The upper end of each cable 41 isconnected to a support padeye 42 which depends from the lower surface ofa transverse coupling plate 43 to which the upper end of each pipelength 40 is also connected. The connection of the upper end of eachpipe length to the coupling plate is via an annular depending flange 44which cooperates closely with the outer circumference of each pipelength, and via an elongate circumference of each pipe length, and viaan elongate tubular blending nipple 45 which cooperates with the innercircumference of each pipe length for a selected distance along the pipefrom its upper end. Each pipe and its blending nipple is connected tothe corresponding outer circumferential flange 44 by a plurality ofbolts passed through the flange, the pipe and the extreme upper end ofthe corresponding blending nipple.

As shown in FIG. 1, blending nipples 45 are not of constant diameteralong their lengths. Rather, proceeding downwardly from their upperends, they are tapered, preferably in a nonlinear manner, so that theyhave a diameter at their lower ends which is less than the innerdiameter of the corresponding pipe length 40.

In view of the connection of weight 17 to the lower end of the pipeassembly and the gimballed suspension of the pipe assembly from the hull11, it is apparent that the pipe assembly tends to hang in a verticalpendulous manner from the hull. By virtue of the details of its designand construction, pipe assembly 12 is adapted to experience minimumbending stresses therein in response to current drag forces applied tothe pipe assembly. The gimballed support of the upper end of the pipeassembly from the hull minimizes the transfer of angular motions of thefloating structure, as by reason of roll or pitch in response to waveaction, to the pipe assembly. To the extent that any angular motions ofa nature such as to induce bending moments in the pipe assembly aretransferred from the floating structure to the pipe assembly, suchmotions, and the loads occasioned thereby, are applied to the upper endof the pipe assembly by the blending nipples in such a manner as tominimize the generation of critical bending stresses within the materialof the pipe assembly. This smooth and acceptable transfer of bendingmoments to the upper end of the pipe assembly is due to the taperedconfiguration of blending nipples 45. That is, such bending moments asmay be applied to the upper end of the pipe assembly from the floatingstructure in use are not applied directly to the extreme upper end ofthe pipe assembly, but rather are transferred to the pipe assemblysubstantially uniformly over a distance corresponding to the length ofthe blending nipples.

A tubular steel transition and ballast section 46 is connected to thelower end of each pipe length 40, as shown in FIG. 1. Each section 46has a portion of its length disposed within the lower terminal portionof each pipe length, and the remainder of the section extends below thelower end of the corresponding pipe length into connection withself-cleaning screen assembly 39 which terminates at its lower end at aninlet flow deflector plate 47. The portions of sections 46 which liewithin pipe lengths 40 are configured in the same manner as blendingnipples 46 to serve the same functions with respect to load transfer asthe blending nipples.

As noted above, pipe assembly 12 is of great length; in the presentlypreferred embodiment, the pipe assembly has a length of 2185 feet fromthe keel of hull 11 to inlet screen 39. The assembly, when in use,passes through at least one ocean current and is subjected tocurrent-induced drag forces. Because of its large diameter, these dragforces are not insignificant and create in the assembly significantbending moments. These movements are largely static, but the pipeassembly will be subjected to dynamic loads due to vortex shedding andmotions of hull 11 in heave and surge, for example. If the pipe assemblywere of steel construction, such loadings would require a very heavyassembly having thick walls in order that stress levels in the wall beheld within acceptable limits. These problems are avoided in pipeassembly 12 by making the assembly flexible along its length. A flexiblepipe assembly deflects with applied loads, rather than stands againstthe loads. Such an assembly can have thinner walls, effectively, than arigid pipe assembly.

Pipe assembly 12 preferably is defined of high density polyethylenewhich has the following properties:

Specific gravity: 0.95±0.002

Poisson's Ratio: 0.3 to 0.5

Thermal Conductivity: 2.5 BTU/hr-ft² -°F.-in.

Coeff. of Thermal Expansion: 9×10⁻⁵ in/in-°F.

Apparent Modulus of Elasticity (psi×10⁴):

    ______________________________________                                        Elapsed Time 46° F. 73° F.                                      ______________________________________                                        One minute   14.5          11.0                                               One hour     7.3           5.3                                                1000 hours   4.5           3.4                                                5 years      4.0           3.0                                                ______________________________________                                    

Each of pipes 40 is 2112 feet long and has a nominal diameter of 48inches; the upper 1386 feet of each pipe length are defined of Series 60pipe and the lower 726 feet are defined of Series 45 pipe. Tensilemembers 41 are defined by lengths of 1.0 inch wire rope brought togetherinto a 2 inch wire rope 48. Stabilizing weight 17 has an immersed weightof 75,000 pounds and is disposed 300 feet below the lower end of thepipe assembly. The lower 65 feet of pipe assembly 12 per se are definedby transition sections 46 and by inlet screen 39.

In such an arrangement, the use of polyethylene to define pipe lengths40 has several advantages which are realizable at relatively low cost.Polyethylene is buoyant in seawater. Positive buoyancy produces simpledeployment, release and recovery procedures, resulting in economies ofcost and shipboard space and reduced complexity. Polyethylene hasdesirable physical properties; it is noncorrosive in seawater, notsubject to endurance limits in the classical sense, and has smoothsurfaces which minimize hydraulic losses, drag forces and biofoulingrates. It is easily and effectively handled and welded. Its flexibilitygreatly reduces dynamic response to induced motions, resulting in lowbending stress levels.

Analysis has shown that a polyethylene pipe assembly of the naturedescribed above can adequately withstand environmental loadings, withoutthe occurrence in the polyethylene of permanent molecular rearrangement,for a period of 5 years if installed in the Pacific Ocean off the westcoast of Hawaii about 18 nautical miles north-west of Keahole Point andslightly south of Kawaihae were the Hawaiian storm current and theHawaiian hurricane would be encountered.

It is noted that the presence of the tensioned cables 41 within the pipelengths 40 enhances the ability of the polyethylene pipe lengths toaccommodate the dynamic loads encountered in such an installation.

Within the lower extremities of the pipe assembly, i.e., within thevertical extent of self-cleaning screen 39, as shown in FIG. 4, theseveral steel cables 41 are brought together to define a single steelcable 48 which extends through deflector plate 47. Cable 48 is reliedupon to carry the immersed weight of bottom weight 17. An acoustic,remotely operable quick-release coupling device 49 is provided in cable48 between the lower end of the pipe assembly and bottom weight 17. Asshown in FIG. 4, screen 39 is composed principally of flexible steelnetting 71 arranged as an inverted conical frustum. The netting iscoupled between an upper carrier plate 72, which is connected to thelower ends of lower transistion sections 46 and which is generally openin way of the ducts defined by such sections save for spiders whichsupport a tubular guide 73 for each of cables 41 coaxially of the lowerend of each section, and a circular bottom closure plate 74. A guide 75is carried centrally of plate 74 to permit weight support cable 48 topass through the plate from within the screen where it is connected, asat 76, to the lower ends of the several cables 41. Cables 41 and 48 passloosely through guides 73 and 75, respectively.

The components of netting 17 are loosely interconnected in the netting,in the manner of chain mail. The netting flow area, porosity, and meshsize are selected to provide significantly greater water flow areathrough the netting than through the several transition sections 46; thenetting does not provide any meaningful restriction on water flow intothe lower end of pipe assembly 12. Preferably the netting surface areais about 5 times the flow area of the pipe assembly, the porosity isabout 80%, and the mesh size is about 6 by 6 inches. The included angleof the conical frustum (one-half angle) is on the order of 20°.

In use of the pipe assembly, the lower end of the assembly willoscillate and move to and fro in response to vortex shedding as theocean current moves past the pipe assembly, for example. This will causethe screen bottom plate to rattle relative to weight support cable 48.Such rattling shakes the netting and, in combination with the angle ofthe netting, frees the netting of any growth thereon and of any matterwhich might be caught thereon by reason the water flow through thenetting.

Steel cables 41 are relied upon to carry the immersed weight of bottomweight 17. It is apparent, therefore, that pipe assembly 12 is of thetensile core, flexible wall construction described in greater detail incopending application Ser. No. 886,904 filed Mar. 15, 1978. Bottomweight 17 is provided for the reasons described in application Ser. No.886,907.

Coupling plate 43 at the upper end of pipe assembly 12 is connected tothe lower end of a hollow, open-ended male pin coupling member 50, aconstant diameter upper portion of which is cooperable with the interiorof female socket member 32. Suitable seals 53 are carried by the femalemember at its tapered sections at the upper and lower ends of itsconstant diameter portion. These seals cooperate with the interior ofthe female member, when these two members are engaged, thereby to definean essentially watertight connection between the members. The sealsbecome effective only on the last increment of motion of the male memberinto the female member. The male and female coupling members ofquick-release connector assembly 15 are held in engagement by aplurality of tether cables 51 which are connected at their upper ends topadeyes carried by frame 30, and at their lower ends to padeyesconnected to the exterior of the male coupling member. A remote releasedevice 52 is provided in the length of each tether cable.

The length of pin member 50 between the locations at which it engagesseal 53 is at least as great as, and preferably is longer than, thediameter of the pin member between the seals. This length is made asgreat as practicable. Such length is desired to make the engagedconnection as insensitive as possible to bending loads applied to it,thereby to isolate the pipe assembly from bending loads, and also toprevent the connection from binding in such a way as to impair theability of the pin member to drop free of the socket member if and whentether cables 51 are severed.

A recovery buoyancy collar 55 is disposed circumferentially about thelower end of male coupling member 50, below that portion of the memberwhich is engaged in the female socket member when connector assembly 15is assembled. The buoyancy of the collar is sufficient that, if itbecomes necessary to jettison the pipe assembly from the hull, thecollar holds the pipe assembly in a vertical attitude after the pipeassembly has dropped to the ocean floor. To facilitate location andrecovery of a jettisoned pipe assembly, a locating acoustic pinger 56and a reestablishment guide post 57 are mounted to the upper extent ofthe buoyancy collar. A recall buoy is disposed in a suitable container58 located on the side of the buoyancy collar adjacent the base of post57. A suitable buoy line 59 is connected from the upper end of post 50to the buoy located within container 58.

A pipe assembly keelhaul latch post 60 is carried by the upper end ofmale pin member 50 and extends upwardly into ball member 33 along theaxis of the pin member.

Buoyancy collar 55, pinger 56, guide post 57, and the recovery buoy andits container 58 are elements of retrieval assembly 16 which is carriedby the upper end of pipe assembly 12 via male pin member 50.

As noted above, pipes 40 preferably are defined of polyethylene;polyethylene is positively buoyant in seawater. In view of this greatlength of the pipe assembly, blending nipples 46 extend along the pipeassembly a distance which is greater than the length of nipples 45 by anamount (about 65 feet) which is selected to cause the combination ofpipes 40 and nipples 46 to have a desired amount of net negativebuoyancy. Inlet screen assembly 39 is also negatively buoyant. Thus, thepipes 40 are under a modest axial tensile load in use, rather than acompressive load which otherwise would result from the positive buoyancyof polyethylene and which could cause the pipes to buckle. The netnegative buoyancy of the combination of pipes 40 and nipples 46 is lessthan the net positive buoyancy of collar 55 and the other structuralencountered at the upper end of the pipe assembly, including male pinmember 50. The net negative buoyancy of weight 17 is greater than thenet positive buoyancy of the portion of the pipe assembly above cable48. For example, in the preferred embodiment described above, the netbuoyancies of these components are as follows:

Male member 50 and upper blending nipples 45--+20,000 lb.

buoyancy collar--+30,000 lb.

polyethylene pipe--+60,000 lb.

lower blending nipples 46--and inlet screen 39---65,000 lb.

cables 41 and 48---17,500 lb.

stabilizing weight 17---17,000 lb.

While the sum of these buoyancies is 87,500 lbs. net negative buoyancy,the pipe assembly itself (exclusive of weight 39 and its suspensioncables) is 5000 lbs. positive.

In view of the foregoing, it is apparent that, when the structuredescribed above is assembled in the manner shown in FIG. 1, a fluid flowduct, having an inlet at screen assembly 39, is defined from the lowerend of the pipe assembly to cold water sump 34 within hull well 18. Thisduct provides a path for the flow of cold water from the deep in theocean upwardly to OTEC facility 10. Deep ocean cold water is induced toflow upwardly along this path by the action of a pump impeller 62 whichis located in the open lower end of a tubular cold water intake duct 63disposed within sump 34. Impeller 62 is mounted on the lower end of anelongate shaft 64, the upper end of which is connected to a drive motor65 which preferably is located at the upper end of well 18. Between thepump impeller and drive motor, an elbow 66 is formed in duct 63 to causecold water drawn into the duct to be discharged into a suitable tank orpiping arrangement (not shown) within the hull of OTEC facility 10.

It will be apparent from an examination of the structure shown in theaccompanying drawings that this structure provides for the ready andconvenient assembly of the overall system, and also for readyjettisoning and recovery of the of the pipe assembly from the floatingstructure, as in the event of an emergency. Assembly and initialconnection of the pipe assembly to hull 11 is shown, in stages in FIGS.5 through 12, and jettisoning and recovery of the pipe assembly is shownin stages in FIGS. 13 through 21.

The pipe assembly preferably is made up and assembled at a shore-basedlocation, as on the beach. The appropriate individual lengths ofpolyethylene pipe are welded together to define the flexible pipeassembly per se; the floatation and retrieval assembly is connected toone end of the piping, and the stabilization weight to the other. Thenegatively buoyant lower end of the pipe assembly and the stabilizingweight are placed on a barge 78 (see FIG. 6), and the rest of the pipeassembly, all of which is buoyant, is allowed to float behind the bargeas it is towed to the installation site where hull 11 has beenprepositioned. As so prepositioned (see FIGS. 5 and 6), the gimbal andball joint assembly 14 will have been lowered in the hull using cable 70and a winch 79 onboard the hull, and the cold water sump 34 will havebeen defined in well 18. When the barge arrives at the hull, the gimbaland ball joint assembly is in position adjacent the hull keel; see FIG.6.

When barge 78 arrives alongside the hull, suitable devices, such asA-frames 80 and winches 81 aboard the hull (see FIG. 7), are rigged andconnections made to the stabilizing weight and the lower end of the pipeassembly for controlled lifting and lowering thereof from the hull. Theload is lifted from the barge and the barge towed out of the way, andthen the load is lowered. As the lower end of the pipe assembly islowered, an auxiliary vessel 82 maintains tension via a pendant line 83on the upper end of the pipe assembly, as by connection to latch post60. The flexibility and positive buoyancy of piping 40 is of benefitduring these processes.

The pipe assembly is lowered until it is vertical and its net negativebuoyancy is supported by pendant line 83; see FIG. 9. The lines toweight 17 and the lower end of the pipe assembly are released, and cable70 is then run down through ball joint 36,37 and to a connection point84 in the pendant line just above latch post 60; see FIG. 10. The loadof the pipe assembly is transferred from the pendant line to cable 70,and the pipe assembly is moved into position below female socket member32. A diver can then release the pendant line from connection point 84.See FIG. 11. The pipe assembly is then drawn by cable 70 into seatingengagement of pin member 50 with socket member 32, and a diver securestether cables 51. Cable 70 is thereafter disconnected from latch post 60to enable the pump assembly, including duct 63 and elbow 66, to belowered into position within sump 34, and to enable the appropriatemechanical connection of the pump assembly to the hull to be made.Preferably, however, the pump assembly is disposed off-center of well18, so that keelhauling of the pipe assembly into connection with thegimbal and ball joint suspension coupling 14 can be accomplished withthe pump assembly in place of sump 34.

If, at any time during the operation of OTEC facility 10, circumstancesshould occur which would require disconnection of the pipe assembly fromthe hull, jettisoning of the pipe assembly is a simple matter. Anappropriate release signal is generated to cause actuation of remotelyoperated quick-release couplings 52 associated with tether cables 51.Couplings 52 operate in such manner that the tether cables partsubstantially simultaneously. As the tether cable part, all support ofthe upper end of the pipe assembly by the hull is terminated. Since thenet buoyancy of the pipe assembly is negative due to the connection ofbottom weight 17 to the pipe assembly, the pipe assembly dropsdownwardly from the hull, carrying male pin member 50 with it butleaving the female socket member and ball element 33 connected to thefloating structure via the gimbal arrangement. Once the bottom weightimpacts the ocean floor, the pipe assembly is maintained in a verticalattitude by reason of the positive buoyancy of collar 35 at the upperend of the pipe assembly. This is shown, in sequence, in FIGS. 13through 16.

The length of cable 48, between inlet screen 39 and stabilizing weight17, is defined so that the pipe assembly cannot damage itself as it isjettisoned, sinks to the ocean floor, and then assumes an essentiallyerect floating state in which it is anchored by the weight and whichfacilitates recover. Upon actuation of quick-release devices 52, thenegatively buoyant pipe assembly drops from socket member 32 andaccelerates to its terminal freefall velocity; in the embodiment forwhich various parameters and characteristics have already been setforth, this velocity is about 8 ft. per second. Weight 17, whichpreferably has a cruciform shape in horizontal cross-section, issuspended sufficiently below the inlet screen by cables 41 and 48 that,once the weight strikes and embeds itself in the sea floor sediment, thenet positive buoyancy of the remaining free-falling structure takeseffect, in combination with viscous drag forces, to slow and stop thedescent of the pipes assembly within the length of cable 48. In thepresently preferred embodiment, cable 48 has a length of 300 feet; thepipe assembly will come to a stop from terminal free-fall velocity,after weight 17 strokes bottom, within approximately 200 feet.Thereafter, the pipe assembly floats substantially vertically erect,displaced only by currents, in a fully submerged state moored by weight17 (see FIG. 16). The lower end of the structure at the upper end ofcable 48 never contacts the sea floor.

A marker buoy stored in container 58 can be released automatically atthe time of jettisoning of pipe assembly from the floating structure,or, as preferred, it can be held captive in its container for subsequentrelease by a suitable remotely controlled release mechanism at a latertime as shown in FIG. 17. The marker buoy provides assistance inreconnecting to the upper end of the jettisoned pipe assembly.

Recovery of the jettisoned pipe assembly from the ocean floor isfacilitated by the presence of remotely operable coupling 49 in theconnection of bottom weight 17 to the lower end of the pipe assembly.That is, if it is desired to recover a jettisoned pipe assembly withoutits bottom weight, a suitable signal is generated to cause releasemechanism 49 to operate to sever the connection between the bottomweight and the pipe assembly, thereby allowing the pipe assembly tofloat to the water surface. On the other hand, if it is desired torecover a jettisoned pipe assembly with its bottom weight, such recoveryis facilitated by the use of reestablishment guide post 57 and keelhaullatch post 60 which provide the means by which a suitable recoverydevice 86 (see FIGS. 18-20) can be securely coupled to the jettisonedpipe assembly for recovery of the same. Marker buoy cable 59 provides ameans for guiding the recovery device into registry with guide and latchposts 57 and 60.

The preferred procedure for recovering a jettisoned pipe assembly, withits stabilizing weight, to hull 11 is shown in FIGS. 17 through 21. AnA-frame 80 is rigged to the side of the hull (FIG. 17) so that buoy line59 can be taken aboard the hull after release of the marker buoy (FIG.16). Line 59 is rigged through the recovery device which is then loweredalong line 59 into contact with guide post 57. The recovery devicepreferably is a Regan reestablishment tool which carries a lift cable 88with a Regan latch at the bottom matable with latch post 60. When therecovery device reaches guide post 57, it cooperates with orientationcam which properly orients the device on the post so that furthermovement of the device downwardly on the post causes the lift line toconnect to post 60 (see FIGS. 18 and 19). The pipe assembly is thenhandled as described above with reference to FIGS. 10 and 11, usingcable 70 and divers, to reconnect pin member 50 to socket member 32. Thefully recovered and reconnected pipe assembly is shown in FIG. 21.

Pipe assembly 12 is made as light as possible consistent with theconditions it must withstand in use. The presence of sump 34 in hull 11contributes to this objective. The normal water level in sump 34 isbelow the load waterline of the hull. A water level in the sump belowsea level is maintained by the operation of pump 65. This differencebetween sea level and sump water level provides a driving headdifferential which is sufficient, in combination with the water flowarea of the pipe assembly, to cause water to flow upwardly through thepipe assembly with the desired average mass flow rate. However, hull 11is subjected to wave action which, besides the rolling and pitchingmotions already noted, can cause the hull to heave, i.e., movecyclically in a vertical direction. The volume of sump 34 below itsnormal water level is several times greater than the volume obtained bythe product of (1) the maximum heave amplitude of the hull (doubleamplitude) and (2) the flow area of the pipe assembly; in the preferredarrangement shown, the sump volume is about 15 times greater than theexpected maximum heave amplitude of 6.6 feet times the duct flow area ofabout 50 square feet. In this way, it is assured that the pipe assemblyis always full of water during heaving motion of the hull, and thedifferential pressures across the walls of the pipe assembly, tending tocollapse the pipe walls, are kept within acceptable limits. This, inturn, allows the pipe assembly to be made lighter than if it wererequired to withstand higher collapse loads.

The operation of pump 65 is controlled, by the assistance of suitablewater level sensors in sump 34, to maintain the desired water level inthe sump.

It will be apparent from the foregoing that all of the vertical loadsbetween the floating structure and the submerged, pendulously supportedpipe assembly are carried by the gimbal arrangement. The ball joint doesnot carry any pipe loads since it is located at the gimbal center ofrotation. The ball joint functions solely as a passage whichaccommodates gimbal motions for carrying cold water across thegimbal-hull interface. All pipe assembly loads are carried directly intothe gimbal structure, thereby minimizing creation of motion-inducedbending stresses in the upper portions of the pipe assembly. The use ofa ball joint seal in combination with the gimbal arrangement allows theprimary cold water pump in the facility to be fixed to the floatingstructure. No other flexible joints, other than the ball joint, arerequired in the cold water duct. Suitable sealing of the ball joint is asimple matter and can be accomplished using standard pressure-loadedsealing techniques which are not intended to provide a 100% effectiveseal. With a small pressure drop (on the order of a 5 foot head) acrossthe ball joint seal, any leakage across the ball joint will be minimal.The use of a male pin at the upper end of the pipe assembly, mating witha female socket below the gimbal arrangement, affords a simple mechanismfor releasing the pipe assembly in an emergency. The bearing and sealrings provided between the male pin and the female socket carry theminimal moment and shear loads which may occur between the gimbal andthe pipe assembly in a completed installation. Vertical loads arecarried by tether cables 51. When the remote release devices (which maybe electric, hydraulic or acoustic) in the tether cables are operated,the male pin carried by the upper end of the pipe assembly can easilyfall free from the gimballed female socket. In use, the gimballedarrangement is located below the water line of the floating hull,thereby itself incurring minimal motions of the hull. The entire gimbalassembly can be raised in its well to above the load waterline of thehull for the purposes of maintenance.

Persons skilled in the art to which this invention pertains willappreciate that the preceding description has been presented withreference to the presently preferred embodiment of the invention asillustrated in the accompanying drawings. It will be understood,however, that the present invention can be manifested in embodimentsdifferent from the described embodiment. The preceding description setsforth the presently known best mode of practicing this invention, butcertainly not all possible modes. Accordingly, workers skilled in theart will readily appreciate that modifications, alterations orvariations in the arrangements and procedures described above may bepracticed without departing from, and while still relying upon, theessential aspects of this invention.

What is claimed is:
 1. An inlet screen for an ocean upwelling pipe andthe like adapted to be disposed substantially vertically in an ocean forflow of sea water through a downwardly facing lower inlet end of thepipe disposed above the ocean floor and upwardly in the pipe, the screencomprising a right conical frustum of flexible loose netting and thelike having an upper end larger in diameter than its lower end, an uppercoupling plate adapted to be connected to the lower end of the pipe andhaving the upper end of the frustum connected thereto, a heavynegatively buoyant closure connected across the lower end of thefrustum, a weight, and a line adapted to be connected to the pipe and topass through the closure substantially centrally thereof for supportingthe weight below the closure above the ocean floor.
 2. Apparatusaccording to claim 1 wherein the netting is defined of metal. 3.Apparatus according to claim 1 wherein the frustum has sufficient areaand the netting has sufficient porosity that the screen has an effectivewater flow area therethrough greater than the flow area of the pipe. 4.Apparatus according to claim 1 wherein the included angle of thefrustum, measured relative to the frustum axis, is on the order of about20°.
 5. An inlet screen useful with a downwardly facing inlet opening toa pipe in an ocean and through which sea water and the like is to flow,the screen comprising a conical frustum of loosely woven wire nettinghaving a larger upper end and a smaller lower end, means for connectingthe frustum upper end to the pipe inlet opening, a heavy negativelybuoyant closure connected across the lower end of the frustum, a weight,a line connectible at one end thereof to the weight for suspending theweight and connectible at its other end to the pipe at or above itslower end, and aperture means defined in the frustum closuresubstantially centrally thereof for passage of the line therethrough andfor loosely surrounding the line when passed therethrough, the linehaving a length sufficient that when it is connected to the weight andto the pipe and passed through the frustum closure the weight issuspended below the closure above the ocean floor.
 6. An inlet screenuseful with a downwardly facing inlet opening to a pipe in an ocean andthrough which sea water and the like is to flow, the screen comprising aconical frustum of loosely woven wire netting having a larger upper endand a smaller lower end, means for connecting the frustum upper end tothe pipe inlet opening, a heavy negatively buoyant closure across thelower end of the frustum, and wherein the included angle of the frustum,measured relative to the frustum axis, is on the order of about 20°.